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diff --git a/open_issues/nice_vs_mach_thread_priorities.mdwn b/open_issues/nice_vs_mach_thread_priorities.mdwn deleted file mode 100644 index 1f4c6ab8..00000000 --- a/open_issues/nice_vs_mach_thread_priorities.mdwn +++ /dev/null @@ -1,429 +0,0 @@ -[[!meta copyright="Copyright © 2010, 2012, 2013 Free Software Foundation, -Inc."]] - -[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable -id="license" text="Permission is granted to copy, distribute and/or modify this -document under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; with no Invariant -Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license -is included in the section entitled [[GNU Free Documentation -License|/fdl]]."]]"""]] - -[[!tag open_issue_gnumach open_issue_glibc]] - -This issue has been known for some time, due to coreutils' testsuite choking -when testing *nice*: [[!debbug 190581]]. - -There has been older discussion about this, too, but this is not yet captured -here. - - -# IRC, freenode, #hurd, 2010-08 - - <pochu> I'm reading Mach and POSIX documentation to understand the priorities/nice problems - <pochu> antrik said it would be better to reimplement everything instead of - fixing the current Mach interfaces, though I'm not sure about that yet - <youpi> uh, so he changed his mind? - <pochu> it seems POSIX doesn't say nice values should be -20..20, but - 0..(2*NZERO - 1) - <youpi> he said we could just change the max priority value and be done - with it :) - <pochu> so we can probably define NZERO to 16 to match the Mach range of - 0..31 - <youpi> s/said/had said previously/ - <antrik> youpi: POSIX is actually fucked up regarding the definition of - nice values - <antrik> or at least the version I checked was - <pochu> antrik: why? this says the range is [0,{NZERO}*2-1], so we can just - set NZERO to 16 AFAICS: - http://www.opengroup.org/onlinepubs/9699919799/functions/getpriority.html - <antrik> it talkes about NZERO and all; making it *look* like this could be - defined arbitrarily... but in other places, it's clear that the standard - 40 level range is always assumed - <antrik> anyways, I totally see no point in deviating from other systems in - this regard. it can only cause problems, and gives us no benefits - <cfhammar> it says NZERO should be at least 20 iirc - <youpi> agreed - <antrik> I don't remember the details; it's been a while since I looked at - this - <antrik> youpi: changing the number of levels is only part of the - issue. I'm not sure why I didn't mention it initially when we discussed - this - <antrik> youpi: I already concluded years ago that it's not possible to - implement nice levels correctly with the current Mach interfaces in a - sane fashion - <antrik> (it's probably possible, but only with a stupid hack like setting - all the thread priorities one by one) - <antrik> youpi: also, last time we discussed this, I checked how the nice - stuff works currently on Hurd; and concluded that it's so utterly broken, - that there is no point in trying to preserve *any* compatibility. I think - we can safely throw away any handling that is alread there, and do it - over from scratch in the most straightforward fashion - <pochu> antrik: I've thought about setting NZERO to 16 and doing exactly - what you've just said to be a hack (setting all the thread priorities one - by one) - <pochu> but there seems to be consensus that that's undesirable... - <pochu> indeed, POSIX says NZERO should be at least 20 - <antrik> pochu: BTW, I forgot to say: I'm not sure you appreciate the - complexity of setting the thread max priorities individually - <pochu> antrik: I don't. would it be too complex? I imagined it would be a - simple loop :) - <antrik> pochu: in order to prevent race conditions, you have to stop all - other threads before obtaining the list of threads, and continue them - after setting the priority for each - <antrik> I don't even know whether it can be done without interfering with - other thread handling... in which case it gets really really ugly - <pochu> antrik: btw I'm looking at [gnumach]/kern/thread.[ch], removing the - priority stuff as appropriate, and will change the tasks code later - <antrik> it seems to me that using a more suitable kernel interface will - not only be more elegant, but quite possibly actually easier to - implement... - <pochu> antrik: apparently it's not that hard to change the priority for - all threads in a task, see task_priority() in gnumach/kern/task.c - <pochu> it looks like the nice test failures are mostly because of the not - 1:1 mapping between nice values and Mach priorities - <marcusb> "Set priority of task; used only for newly created threads." - <marcusb> there is a reason I didn't fix nice 8 years ago - <marcusb> ah there is a change_threads option - <pochu> marcusb: I'm not sure that comment is correct. that syscall is used - by setpriority() - <marcusb> yeah - <marcusb> I didn't read further, where it explains the change_threads - options - <marcusb> I was shooting before asking questions :) - <marcusb> pochu: although there are some bad interactions if max_priorities - are set per thread - <antrik> pochu: maybe we are talking past each other. my point was not that - it's hard to do in the kernel. I was just saying that it would be painful - to do from userspace with the current kernel interface - <pochu> antrik: you could still use that interface in user space, couldn't - you? or maybe I'm misunderstanding... - <pochu> cfhammar, antrik: current patch: - http://emilio.pozuelo.org/~deb/gnumach.patch, main issue is probably what - to do with high-priority threads. are there cases where there should be a - thread with a high priority but the task's priority shouldn't be high? - e.g. what to do with kernel_thread() in [gnumach]/kern/thread.c - <pochu> i.e. if tasks have a max_priority, then threads shouldn't have a - higher priority, but then either we raise the task's max_priority if we - need a high-prio thread, or we treat them specially (e.g. new field in - struct thread), or maybe it's a non-issue because in such cases, all the - task is high-prio? - <pochu> also I wonder whether I can kill the processor set's - max_priority. It seems totally unused (I've checked gnumach, hurd and - glibc) - <pochu> (that would simplify the priority handling) - <cfhammar> pochu: btw what does your patch do? i can't remember what was - decided - <pochu> cfhammar: it moves the max_priority from the thread to the task, so - raising/lowering it has effect on all of its threads - <pochu> it also increases the number of run queues (and thus that of - priority levels) from 32 to 40 so we can have a 1:1 mapping with nice - values - <pochu> cfhammar: btw don't do a full review yet, just a quick look would - be fine for now - <neal> why not do priorities from 0 to 159 - <neal> then both ranges can be scaled - <neal> without loss of precision - <pochu> neal: there would be from Mach to nice priorities, e.g. a task with - a priority of 2 another with 3 would have the same niceness, though their - priority isn't really the same - <neal> pochu: sure - <neal> pochu: but any posix priority would map to a current mach priority - and back - <neal> sorry, that's not true - <neal> a posix priority would map to a new mach priority and bach - <neal> and a current mach priority would map to a new mach priority and - back - <neal> which is I think more desirable than changing to 40 priority levels - <pochu> neal> and a current mach priority would map to a new mach priority - and back <- why should we care about this? - <neal> to be compatible with existing mach code - <neal> why gratutiously break existing interfaces? - <pochu> they would break anyway, wouldn't them? i.e. if you do - task_set_priority(..., 20), you can't know if the caller is assuming old - or new priorities (to leave it as 20 or as 100) - <neal> you add a new interface - <neal> you should avoid changing the semantics of existing interfaces as - much as possible - <pochu> ok, and deprecate the old ones I guess - <neal> following that rule, priorities only break if someone does - task_set_priority_new(..., X) and task_get_priority () - <neal> there are other users of Mach - <neal> I'd add a configure check for the new interface - <neal> alternatively, you can check at run time - <pochu> well if you _set_priority_new(), you should _get_priority_new() :) - <neal> it's not always possible - <pochu> other users of GNU Mach? - <neal> you are assuming you have complete control of all the code - <neal> this is usually not the case - <neal> no, other users of Mach - <neal> even apple didn't gratuitously break Mach - <neal> in fact, it may make sense to see how apple handles this problem - <pochu> hmm, I hadn't thought about that - <pochu> the other thing I don't understand is: "I'd add a configure check - for the new interface". a configure check where? in Mach's configure? - that doesn't make sense to me - <neal> any users of the interface - <pochu> ok so in clients, e.g. glibc & hurd - <neal> yes. - <antrik> neal: I'm not sure we are winning anything by keeping - compatibility with other users of Mach... - <antrik> neal: we *know* that to make Hurd work really well, we have to do - major changes sooner or later. we can just as well start now IMHO - <antrik> keeping compatibility just seems like extra effort without any - benefit for us - <guillem> just OOC have all other Mach forks, preserved full compatibility? - <neal> guillem: Darwin is pretty compatible, as I understand it - <antrik> pochu: the fundamental approach of changing the task_priority - interface to serve as a max priority, and to drop the notion of max - priorities from threads, looks fine - <antrik> pochu: I'm not sure about the thread priority handling - <antrik> I don't know how thread priorities are supposed to work in chreads - and/or pthread - <antrik> I can only *guess* that they assume a two-stage scheduling - process, where the kernel first decides what process to run; and only - later which thread in a process... - <antrik> if that's indeed the case, I don't think it's even possible to - implement with the current Mach scheduler - <antrik> I guess we could work with relative thread priorities if we really - want: always have the highest-priority thread run with the task's max - priority, and lower the priorities of the other threads accordingly - <antrik> however, before engaging into this, I think you should better - check whether any of the code in Hurd or glibc actually uses thread - priorities at all. my guess is that it doesn't - <antrik> I think we could get away with stubbing out thread priority - handling alltogether for now, and just use the task priority for all - threads - <antrik> I agree BTW that it would be useful to check how Darwin handles - this - <pochu> btw do you know where to download the OS X kernel source? I found - something called xnu, but I?m not sure that's it - <antrik> pochu: yeah, that's it - <antrik> Darwin is the UNIX core of OS X, and Xnu is the actual kernel... - <pochu> hmm, so they have both a task.priority and a task.max_priority - <neal> pochu: thoughts? - <pochu> neal: they have a priority and a max_priority in the task and in - the threads, new threads inherit it from its parent task - <pochu> then they have a task_priority(task, priority, max_priority) that - can change a task's priorities, and it also changes it for all its - threads - <neal> how does the global run queue work? - <pochu> and they have 128 run queues, no idea if there's a special reason - for that number - <pochu> neal: sorry, what do you mean? - <neal> I don't understand the point of the max_priority parameter - <pochu> neal: and I don't understand the point of the (base) priority ;) - <pochu> the max_priority is just that, the maximum priority of a thread, - which can be lowered, but can't exceed the max one - <pochu> the (base) priority, I don't understand what it does, though I - haven't looked too hard. maybe it's the one a thread starts at, and must - be <= max_priority - <antrik> pochu: it's clearly documented in the manual, as well as in the - code your initial patch changes... - <antrik> or do you mean the meaning is different in Darwin?... - <pochu> I was speaking of Darwin, though maybe it's the same as you say - <antrik> I would assume it's the same. I don't think there would be any - point in having the base vs. max priority distinction at all, except to - stay in line with standard Mach... - <antrik> at least I can't see a point in the base priority semantics for - use in POSIX systems... - <pochu> right, it would make sense to always have priority == max_priority - ... - <pochu> neal: so max_priority is that maximum priority, and priority is the - one used to calculate the scheduled priority, and can be raised and - lowered by the user without giving special permissions as long as he - doesn't raise it above max_priority - <pochu> well this would allow a user to lower a process' priority, and - raise it again later, though that may not be allowed by POSIX, so then we - would want to have max_priority == priority (or get rid of one of them if - possible and backwards compatible) - <antrik> pochu: right, that's what I think too - <antrik> BTW, did I bring up handling of thread priorities? I know that I - meant to, but I don't remember whether I actually did... - <pochu> antrik: you told me it'd be ok to just get rid of them for now - <pochu> so I'm more thinking of fixing max_priority and (base) priority and - leaving thread's scheduling priority as it currently is - <pochu> s/so/though/ - <antrik> pochu: well, my fear is that keeping the thread priority handling - as ist while changing task priority handling would complicate the - changes, while giving us no real benefit... - <antrik> though looking at what Darwin did there should give you an idea - what it involves exactly... - <pochu> antrik: what would you propose, keeping sched_priority == - max_priority ? - <pochu> s/keeping/making/ - <antrik> yes, if that means what I think it does ;-) - <antrik> and keeping the priority of all threads equal to the task priority - for now - <antrik> of course this only makes sense if changing it like this is - actually simpler than extending the current handling... - <antrik> again, I can't judge this without actually knowing the code in - question. looking at Darwin should give you an idea... - <pochu> I think leaving it as is, making it work with the task's - max_priority changes would be easier - <antrik> perhaps I'm totally overestimating the amount of changes required - to do what Darwin does - <antrik> OTOH, carrying around dead code isn't exactly helping the - maintainability and efficiency of gnumach... - <antrik> so I'm a bit ambivalent on this - <antrik> should we go for minimal changes here, or use this occasion to - simplify things?... - <antrik> I guess it would be good to bring this up on the ML - <cfhammar> in the context of gsoc i'd say minimal changes - <pochu> there's also neal's point on keeping backwards compatibility as - much as possible - <neal> my point was not backwards compatibility at all costs - <antrik> I'm still not convinced this is a valid point :-) - <neal> but to not gratutiously break things - <antrik> neal: well, I never suggested breaking things just because we - can... I only suggested breaking things to make the code and interface - simpler :-) - <antrik> I do not insist on it though - <neal> at that time, we did not know how Mac did it - <antrik> I only think it would be good to get into a habit that Mach - interfaces are not sacred... - <neal> and, I also had a proposal, which I think is not difficult to - implement given the existing patch - <antrik> but as I said, I do not feel strongly about this. if people feel - more confident about a minimal change, I'm fine with that :-) - <antrik> neal: err... IIRC your proposal was only about the number of nice - levels? we are discussing the interface change necessary to implement - POSIX semantics properly - <antrik> or am I misremembering? - <pochu> antrik: he argues that with that number of nice levels, we could - keep backwards compatibility for the 0..31 levels, and for 0..39 for - POSIX compatibility - <antrik> pochu: yes, I remember that part - <neal> antrik : My suggestion was: raise the number of nice levels to 160 - and introduce a new interface which uses those. Adjust the old interface - to space by 160/32 - <antrik> neal: I think I said it before: the problem is not *only* in the - number of priority levels. the semantics are also wrong. which is why - Darwin added a max_priority for tasks - <neal> what do you mean the semantics are wrong? - <neal> I apologize if you already explained this. - <antrik> hm... I explained it at some point, but I guess you were not - present at that conversation - <neal> I got disconnected recently so I likely don't have it in backlog. - <antrik> in POSIX, any process can lower its priority; while only - privileged processes can raise it - <antrik> Mach distinguishes between "current" and "max" priority for - threads: "max" behaves like POSIX; while "current" can be raised or - lowered at will, as long as it stays below "max" - <antrik> for tasks, there is only a "current" priority - <antrik> (which applies to newly created threads, and optionally can be set - for all current threads while changing the task priority) - <antrik> glibc currently uses the existing task priorities, which leads to - *completely* broken semantics - <antrik> instead, we need something like a max task priority -- which is - exactly what Darwin added - <neal> yes - <antrik> (the "current" task priority is useless for POSIX semantics as far - as I can tell; and regarding thread priorities, I doubt we actually use - them at all?...) - <cfhammar> where does a new thread get its initial max_priority from? - <antrik> cfhammar: from the creator thread IIRC - <pochu> yes - - -## IRC, freenode, #hurd, 2010-08-12 - - <pochu> my plan is to change the number of priority levels and the - threads/tasks priority handling, then add new RPCs to play with them and - make the old ones stay compatible, then make glibc use the new RPCs - - -# IRC, freenode, #hurd, 2012-12-29 - - <braunr> and, for a reason that i can't understand, there are less - priorities than nice levels, despite the fact mach was designed to run - unix systems on top of it .. - <youpi> btw, didn't we have a plan to increase that number? - <braunr> i have no idea - <braunr> but we should :) - <youpi> I remember some discussion about it on the list - - -## IRC, freenode, #hurd, 2012-12-31 - - <youpi> braunr: btw, we *do* have fixed the nice granularity - <youpi> +#define MACH_PRIORITY_TO_NICE(prio) ((prio) - 20) - <youpi> in the debian package at least - <youpi> ah, no - <youpi> it's not applied yet - <youpi> so I have the patch under hand, just not applied :) - <braunr> but that's a simple shift - <braunr> the real problem is that there aren't as many mach priorities as - there are nice levels - <youpi> that's not really a problem - <youpi> we can raise that in the kernel - <youpi> the problem is the change from shifted to unshifted - <youpi> that brings odd nice values during the upgrade - <braunr> ok - <braunr> i hope the scheduler code isn't allergic to more priorities :) - - -## IRC, freenode, #hurd, 2013-01-02 - - <braunr> pochu: i see you were working on nice levels and scheduling code - some time ago - <braunr> pochu: anything new since then ? - <pochu> braunr: nope - <braunr> pochu: were you preparing it for the gsoc ? - <pochu> braunr: can't remember right now, either that or to fix a ftbfs in - debian - <youpi> iirc it's coreutils which wants proper nice levels - - -# IRC, OFTC, #debian-hurd, 2013-03-04 - - <Steap> Is it not possible to set the priority of a process to 1 ? - <Steap> these macros: - <Steap> #define MACH_PRIORITY_TO_NICE(prio) (2 * ((prio) - 12)) - <Steap> #define NICE_TO_MACH_PRIORITY(nice) (12 + ((nice) / 2)) - <Steap> are used in the setpriority() implementation of Hurd - <Steap> so setting a process' priority to 1 is just like setting it to 0 - <youpi> Steap: that has already been discussed to drop the *2 - <youpi> the issue is mach not supporting enough sched levels - <youpi> can be fixed, of course - <youpi> just nobody did yet - -GNU Mach commit 6a234201081156e6d5742e7eeabb68418b518fad (and commit -6fe36b76f7983ec9a2e8a70420e431d54252442e). - - -## IRC, OFTC, #debian-hurd, 2013-03-07 - - <braunr> youpi: btw, why did you increase the number of priorites to 50 ? - <youpi> for the nice levels - <braunr> and probably something more, there are only 40 nice levels - <youpi> yes, the current computation leaves some margin - <youpi> so I preferred to keep a margin too - <braunr> ok - <youpi> e.g. for the idle thread, etc. - <braunr> or interrupt threads - <youpi> yep - <braunr> i see the point, thanks - <tschwinge> Is the number of 40 specified by POSIX (or whatever) or is that - a Linuxism? - <braunr> good question - <braunr> posix is weird when it comes to such old unixisms - <braunr> there is a NZERO value, but i don't remember how it's specified - <youpi> it's at least 20 - <tschwinge> (I don't object to the change; just wondered. And if practice, - you probably wouldn't really need more than a handful. But if that - change (plus some follow-up in glibc (?) improves something while not - adding a lot of overhead, then that's entirely fine, of course.) - <braunr> "A maximum nice value of 2*{NZERO}-1 and a minimum nice value of 0 - shall be imposed by the system" - <braunr> NZERO being 20 by default - <youpi> and 20 is the minimum for NZERO too - <braunr> hm, not the default, the minimul - <braunr> minimum - <braunr> yes that's it - <braunr> ok so it's actually well specified - <tschwinge> Aha, I see (just read it, too). So before that change we - simply couldn't satisfy the POSIX requirement of (minimum) NZERO = 20, - and allowing for step-1 increments. Alright. - <youpi> yep - <youpi> thus failing in coreutils testsuite |